Maritime oil emulsions are usually present in ballast tanks and bilges of ships. Various proposals have been put forward for separating the water from the oil and if the residual water is to be reused or disposed of in sewers or waterways, the degree of oil removal must be such that the residual water contains less than 10 parts per million of hydrocarbon oil.
The "Oil/Water Separation State-of-the-Art" publication prepared for Industrial Environment Research Laboratories in Cincinnati, Ohio by Rutgers State University, New Brunswick, N.J., United States Department of Commerce National Technical Information Service P.B. - 280755 is a thorough review of the problem and of updated separation procedures.
Few known procedures are successful when the oil emulsion is stabilised by a detergent especially when the maximum allowable oil content of the separated water is 10 parts per million. Complex chemical, physical and biological methods, often all three in sequence, are needed if the water must also meet rigid environmental specifications for detergents.
Oil/water/detergent blends are found in ship bilges and ballast tanks. The detergent can enter the ship system from deck cleaners, oil dispersers, laundry wastes, fire foams and deliberate addition to aid cleaning of oil storage tanks. Moreover, a detergent is an essential ingredient of industrial cutting and cooling emulsions and associated rinsing liquors, used to repair marine engines.
In all these uses, the detergent concentration and the chemical nature of the detergent are very variable due to sporadic need or uncontrolled dilution with fresh or salt water.
Ship requirements indicate the need for an on-board system so that water, substantially free of oil, but still containing biodegradable detergents can be released at sea, rather than be brought to shore where dockside water often cannot accept the detergents and other soluble contaminants which may arise from chemical and biological attack on the oil in the bilges. For example, poisonous hydrogen sulphide may be formed and, if so, needs immediate removal along with other soluble biological products while still at sea.
Recently, ultra-filtration has been used with limited success for these detergent stabilised emulsions. In principle, the oil is retained by its inability to flow through the very fine hydrophilic pores of the ultra-filter membrane whilst water passes under quite low pressure. The oil retention is by a combination of geometry and surface tension. The oil breakthrough pressure, P, is given by: ##EQU1## where: s is the oil/aqueous continuous phase interfacial tension,
a is the contact angle of the continuous phase of the pore fluid with the port wall, PA1 d is the pore diameter.
Detergents lower the oil/aqueous interfacial tension s and cause breakthrough of oil at even low pressures. The interaction is complex since the detergent forms micellar structures with itself and with the oil. The critical micellar concentration depends on surfactant composition, on pH, on salts and on temperature. Special problems of anionic detergents in sea water are detailed later.
A major problem with all oil/water ultrafilters arises because they are used in the "cross-flow" mode--that is, the feed flows across the ultrafilter where some water permeates, but most of the emulsion (now richer in oil) returns to the feed. Thus, the feed oil concentration continuously rises which always reduces permeation rate but this is not the worst effect. Most ultrafilters also show some rejection of soluble anionic detergent so that the detergent concentration also rises rapidly in the diminishing recycle aqueous phase.
Hence all the water cannot be substantially removed before oil breaks through. Even ultrafilters with pores rejecting over 99% of ovalbumin of molecular weight 45,000 cannot bring the oil concentration above 50% in the presence of most anionic detergents in hard water such as seawater.
A hitherto unappreciated set of effects further complicates the conventional use of hydrophilic ultrafilters to separate clear water from oils mixed with sea-water in the presence of the common anionic detergents. Sea-water contains 410 parts per million of calcuim ion and this calcium partly precipitates a greasy calcium salt when more than 40 to 100 parts per million (depending on solubility in any oil present) of the usual dodecylbenzenesulfonate ion are present.
Since the usual detergent and sea water mixtures contain up to 500 parts per million of sodium dodecylbenzenesulfonate, a considerable precipitate of calcium dodecylbenzenesulfonate forms and collects at the water/oil interface. The solubility of the calcium salt in oily paraffins is only 70 parts per million and only 220 parts per million in a good aromatic solvent such as toluene. Hence there is no solvent present for the calcium salt in practical oil water separation unless huge quantities of free aromatic oil are present.
Furthermore the conventional hydrophilic ultrafilters sold for oil/water separation are strongly charged on their surfaces by sulfonate groups in order to render them hydrophilic. An unfortunate side effect arises in that Donnan effects reject some of the dodecylbenzenesulfonate ions, thus quickly increasing their concentration and precipitating greasy calcium dodecylbenzenesulfonate, even from solutions which were initially too dilute to precipitate.
The accumulation of greasy calcium salt leads to such blockage of the ultrafilter that normal backwashing with permeate is ineffective. It should be noted that ultrafilters backwash at permeation velocities of less than one meter per hour so that no jet cleaning action can be involved to remove tenacious blockages.
Calcium salts are difficult to remove from hydrophilic surfaces since they tend to adhere to the water phase rather than the oil phase. They thus adhere to the hydrophilic membranes of all present commercial oil/water ultrafilter separators.